1. Field of the Invention
The present invention relates to an optical fiber Mach-Zehnder interferometer optical filter, and in particular to a multiwavelength transmission type optical filter, which is capable of controlling a transmission wavelength, using a stabilized optical fiber Mach-Zehnder interferometer.
2. Description of the Conventional Art
Recently, the wavelength division multiplex optical communication technique has a big attention as a terabit class optical communication technique. In this technique, a device such as an optical filter, an optical wavelength multiplexer, a demulfiplexer, etc. capable of controlling light in a wavelength domain is used as an important component. In view of this element, the AWG(Arrayed Waveguide Grating) disclosed in the end of 1980s has been used as a device capable of obtaining a desired performance as a wavelength division multiplex device.
In addition, methods in which an optical fiber interferometer having a transmission characteristic which periodically varies in accordance with a wavelength is used as a wavelength division multiplex device have been studied.
As a result of the above-described study, a multiwavelength transmission type optical filter using a Fabry-Perot(FP) interferometer for controlling a transmission center wavelength is disclosed in the U.S. Pat. No. 5,062,684 entitled "Optical fiber filter". In this patent, a stabilizing apparatus is not used for external environment such as a temperature variation, etc. by using a FP interferometer in which an optical filter is formed of a ferrul, a support and a piezo-electric device. In the U.S. Pat. No. 5,289,552 entitled "Temperature compensated fiber Fabry-Perot filters", a FP interferometer filter stable for a temperature variation and capable of controlling the transmission wavelength is disclosed. In this patent, the construction of the interferometer is very similar to that in the previous patent. In addition, in this patent, the temperature variation is compensated by changing the position for fixing the optical fiber. In the above-described two patents, the construction of the interferometer is complicated.
In addition, in the U.S. Pat. No. 5,295,205 entitled "Environmentally stable monolithic Mach-Zehnder device", a multiwavelength transmission type interferometer filter using a stable optical fiber Mach-Zehnder interferometer having a small variation of the transmission center wavelength according to the temperature variation is disclosed. In this patent, in order to improve the problem that the transmission center wavelength is varied due to a temperature, the Mach-Zehnder interferometer is formed by inserting two strands of optical fibers into a glass tube. In this patent, the construction is also complicated. In addition, it is impossible to stably control the transmission wavelength.
Namely, since the optical fiber interferometer is sensitive to an external environment such as a temperature variation, a vibration, noise, etc, it should be stabilized for an actual use in the industry.
As a method for stabilizing the interferometer, an active phase tracing method was disclosed. In this method, two outputs of the interferometer are received using two optical detectors for thereby obtaining a difference therebetween and integrating the same, so that an integration signal is obtained. The thusly obtained integration signal is fed to an optical fiber phase modulator(fiber stretcher) formed in one optical path for thereby constantly maintaining the optical path difference of the interferometer.
However, in the above-described active phase tracing method, the interferometer may be stabilized, but it is impossible to continuously control the transmission wavelength.